This invention is a new approach for monitoring the deposition of a rugate filter. It is an optical monitor using as few as one cycle reflectance (or transmittance) of the deposited rugate refractive index. This approach yields all of the three basic parameters of the rugate cycle: the average index, the peak-to-peak index variation, and the wavelength position of the rugate stop band. The basic features of this monitor are described, but not necessarily all the implementations of the use of this method. Once the fundamental concepts are understood several embodiments become obvious to one skilled in thin film filter deposition. Optical interference filters typically consist of alternating thin films of high and low refractive index. The thicknesses of these layers are important in order to achieve the desired spectral properties of the filter, such as its reflectance and transmittance. The design thicknesses are important to achieve in the filter in order to obtain the spectral features of the filter design. Typically layer thicknesses are monitored. Sometimes when the layer being deposited has a known and stable deposition rate, the deposition may be terminated by using a clock to time the deposition duration. Many factors determine the layer thickness accuracy. Layer source material may change its evaporation rate due to source depletion or surface changes, for example. In this case layer thickness monitors are used to terminate the deposition to achieve the required layer thickness. The current state of the art in the monitoring of thin films is given in books such as Thin-Film Optical Filters, by H. A. Macleod, Taylor & Francis Group (2001); Optical Coating Technology by Philip W. Baumeister, SPIE Press (2004); and Practical Monitoring and Control of Optical Thin Films by Ronald R. Willey, Willey Optical, Consultants (2007).
There are two types of thin film monitors in general use: crystals and optical monitors. Crystals measure the weight of the deposition on a small quartz crystal by looking at the change in frequency of its resonant oscillations. Crystals can become noisy, especially after accumulating much material, and become less reliable. Optical monitors use wave front interference of a projected light beam on a depositing part to deduce its optical thickness. Optical thickness is the product of the physical thickness of the layer and its refractive index. When the refractive index of the deposited material is known (as is typically the case for most optical filters using alternating layers of high and low refractive index layers), the physical thickness is inferred. Some high performance filters such as those used in telecom applications (very narrow band filters) are manufactured using optical monitoring.
Rugate Filters
Neither of the current types of deposition monitors are particularly effective when depositing rugate filters. The reason is that the refractive index in a rugate filter is not fixed, but is constantly changing. A rugate filter consists of a sinusoidal refractive index profile, rather than alternating high and low refractive index layers using two materials. When z is the optical thickness depth of a rugate filter, the refractive index for a rugate filter is,n(z)=na+0.5np sin(2πz/λr),  (1)where na is the average refractive index, np is the full amplitude of the sinusoidal index variation, and λr is the wavelength position of the resulting rugate stop band. A filter having a varying refractive index is sometimes called a gradient index optical filter. The sinusoidal refractive index is achieved by co-evaporating a high and a low refractive index material while carefully adjusting the deposition rates. This is described in U.S. Pat. No. 4,934,788, Deposition of gradient index coatings using coevaporation with rate control, William H. Southwell inventor, and U.S. Pat. No. 5,000,575, Method of fabricating gradient index optical films, William H. Southwell and Randolph L. Hall inventors.
The prior art of deposition of rugate filters includes a method using a broad band optical monitor to obtain the current total optical thickness of the deposition. This total optical thickness is used to adjust the next incremental refractive index level using Eq. (1). This is described in U.S. Pat. No. 5,425,964, Deposition of multiple layer thin films using a broad band spectral monitor, William H. Southwell and Randolph L. Hall inventors. Adjusting the sinusoidal refractive index on the basis of the current total optical thickness assures the filter's period as seen in Eq. (1). This means that the broad band spectral monitor is good for achieving the correct wavelength placement of the rugate stop band.
Shortcomings of the Prior Art
Although the optical thickness monitor of the previous art controls λr (the wavelength position of the rugate line), it reveals nothing about the average index na and sinusoidal amplitude np. These parameters of the rugate refractive index are important because na determines the spectral shift with angle of incidence and np/na determines the band width of the rugate line. [See “Spectral response calculations of rugate filters using coupled-wave theory,” by W. H. Southwell, Journal of the Optical Society of America A, Vol. 5, pp 1558-1564 (1988)]
A rugate line centered at λr at normal incidence will shift to lower wavelengths with increasing angle of incidence according to,λ=λr{1−(sin θ/na)2}1/2,  (2)where θ is the angle of incidence of the light. The bandwidth B of the rugate line is given by,B=Δλ/λ=np/(2na).  (3)
Filters fabricated using the optical monitor often have the correct line position but will have incorrect bandwidth and angle sensitivities.
What is needed is a monitor that will reveal the rugate average index na as well as the sinusoidal amplitude np in addition to the period of the sinusoidal refractive index variation.